In a variety of thermal equipment having a heat source, a thermistor is used as a temperature sensor which detects and measures a temperature of the heat source and controls the equipment.
For example, a temperature sensor using a thermistor composition (thermistor element) as a temperature sensing element is used to measure an exhaust gas (heat source) of a vehicle or the like equipped with a diesel engine. Generally, in such a temperature sensor, a metallic electrode is formed in the thermistor composition, and a lead wire (wiring) is connected to the electrode.
In this case, noble metals such as platinum (Pt) and rhodium (Rh) which have excellent corrosion resistance and can be electrically conducted even under a high temperature environment are used for the lead wire. However, these noble metals are expensive and not advantageous from the viewpoint of manufacturing cost.
Meanwhile, a temperature sensor element in which a diffusion layer is formed on an interface between a thermistor and a metal electrode bonded to a surface of the thermistor to ensure junction reliability between the thermistor and the metal electrode and also to ensure ohmic contact has been proposed (refer to Patent Literature 1).
Further, a thermistor in which an electrode has a two-layer structure including an element electrode and a cover electrode and an element electrode containing ruthenium oxide (RuO2) is stacked on a thermistor element has been proposed (refer to Patent Literature 2 and Patent Literature 3).
Further, a glass sealed type thermistor in which a thermistor element and a lead wire formed of a nickel coated Dumet wire or an Fe/Cr alloy wire are connected via an electrode is known (refer to Patent Literature 4).